Previous research has shown that the immature nervous system of some mammals will adapt much more completely to damage than will the adult brain, with functional specificity of compensatory neural connections in some way being maintained. In particular, rats that are subjected to ablation of one motor cortex neonatally will develop compensatory corticospinal (CS) projections from the surviving motor cortex, so that it now innervates the ipsilateral as well as the contralateral spinal cord. As a result, some functional control is retained over the ipsilateral limb, as well as near-normal control over the contralateral limb. There is essentially nothing known, however, about the growth patterns of this compensatory (ipsilateral) projection, nor of the manner in which its formation alters the normal pattern of somatotopic organization within the surviving motor cortex. Moreover, it has yet to be clearly established that the remaining control of the limb ipsilateral to the intact cortex is indeed mediated by the presumed compensatory CS projection, rather than by a similar compensatory growth in ipsilaterally acting, extrapyramidal pathways. In the present research, we will use a combination of intracortical and intraspinal microstimulation techniques, coupled with antero- and retrogradely transported anatomical markers, to study the growth pattern of the compensatory CS projection, its functional role in the adult, and the manner in which its formation is accompanied by changes in the somatotopic organization of the surviving motor cortex. New data will also be obtained on the development of somatotopic specificity in the CS system of the normal neonatal rat. These studies will be among the first aimed at a description of developing, point-to-point projections in the CS system, and will be somewhat unique in that both anatomical and electrophysiological methods will be used. Moreover, they will aid in the evaluation of this animal model as a fruitful one for further study of the development of specificity of connections in central motor pathways, and the extent to which they can be remodeled by plastic or adaptive changes at the early postnatal stage of development.